At the battle zone beat your rival 100 times and you will be able to enlarge it. heres some more stuff:

beat him 150 times:get a gorgeous gem
beat him 200 times:get a pokemon trophy (bronze)
beat him 250 times:get a pokemon trophy (silver)
beat him 300 times:get a pokemon trophy (gold)
beat him 1000 times:you unlock the hoenn region-you can go there and explore
beat him 1500 times: unlock johto region
beat him 2000 times: unlock kanto region -yes this seems impossible but its not. just go every few minutes and challenge him. oh yeah- if you beat him over 10000 times your game will get a serious glitch